1. Technical Field
The present invention relates generally to cryocoolers, and more particularly, to two-stage cryocoolers having a Stirling/pulse tube hybrid configuration.
2. Description of the Related Art
Multistage cryocoolers are of fundamental interest for many applications in which cryogenic cooling is required. For example, some applications require the simultaneous cooling of two objects to cryogenic, but different, temperatures. In the case of a long wave infrared sensor, for instance, the focal plane assembly may require an operating temperature of around 40 K., while the optics may need to be maintained at a different temperature, such as about 100 K. One approach for such situations is to use a single-stage cooler and extract all of the refrigeration at the coldest temperature. However, this is thermodynamically inefficient. Another approach is to use two single-stage cryocoolers with one each at the two temperature reservoirs. This approach has the disadvantage of being expensive and large in size. A better approach that has been done in the past is to use a two-stage cryocooler with the first-stage cooling of the higher operating temperature component, and the second stage cooling the lower operating temperature component. Multistage cryocoolers are generally more efficient than single-stage coolers, because a portion of the internal parasitic thermal losses can occur at higher temperatures, thus producing less entropy generation.
Space-based cryocooler requirements put a high premium upon small volume, low weight, and high reliability. One approach that has been taken in the past is to use a two-stage cryocooler with a first-stage Stirling cryocooler, and a second-stage pulse tube cryocooler. Such an arrangement provides high efficiency, long life, and compact size for the system. Examples of such systems may be found in co-owned U.S. Pat. Nos. 6,167,707 and 6,330,800. Other and viable multistage configurations include multistage Stirling cryocoolers and multistage pulse tubes.
While some success has been achieved with the prior approaches described above, it will be appreciated that improvements may be desirable, as is generally the case in the vast majority of technical areas.